1. Field of the Invention
The present invention relates to a method for the production of spherical shaped granules of low melting point, corrosive, subliming substances, and to an apparatus for minimizing sublimation during their manufacture.
2. Description of Prior Art
The most frequently used granulation processes are: prilling, pan- drum- or fluidized bed- granulation, and compaction granulation. The last two techniques are not of practical value if corrosive and sublimable substances are to be granulated. For this reason only prilling will be analyzed.
Prilling is a well known process which is extensively used in the fertilizer industry for obtaining spherical shaped granules. For instance, this technique is commonly practiced in the saltpeter industry. It can be used if it is possible to melt the material, which, in the molten condition, is discharged into an enclosed space, known as a prilling tower, through nozzles or perforated plates. If the temperature difference between the atmospheric air and the melt is sufficiently high, the droplets can be cooled and solidified solely with the use of a countercurrent air stream. The solid granules are generally received in a chute, or in a liquid, if contamination of the substance with the liquid is not harmful or if the substance is not soluble in the liquid.
If the temperature of the melt is relatively low, or if the height of the prilling tower is limited, cooling can be accelerated by using a spray nozzle on the inside of the enclosed space, which sprays a coolant having a boiling point lower than the melting point of the prilling substance. The liquid spray is directed onto the molten droplets which are thus cooled by means of the latent heat of vaporization of the liquid. This process is described in U.S. Pat. No. 4,552,566.
If the material readily sublimes and the vapors are corrosive, however, a system needs to be developed which minimizes sublimation. Moreover, if the collection of these vapors is an expensive process, sublimation must be kept as low as possible. This has not been achieved by the present state of the art.